Struts for ocean platforms are made in prefabricated sections, to be assembled together during final deployment. The assembly process must be done quickly, since the predictable weather window, in which deployment must be completed, is, in those deep oceans of the world where resouces are thought to exist, hardly ever more than two or three days. There is no time for a vigorous inspection procedure, during assembly. There is a need therefore to inspect the strut periodically during its service life.
For this reason, it is preferred that the strut be hollow, so that an inspection device may be lowered internally down the strut.
To describe it in more detail, the inspection procedure comprises sending an ultrasonic send-and-receive transducer down the strut. The transducer moves down in a predetermined spiral, and transmits a trace that can be recorded, and compared with previous traces, to give an early warning of possible weakening of the strut. Ultrasonic measurements of the kind required can only be carried out reliably and economically if the surface of the metal being tested is under water, since ultrasonic signals in air are swamped with noise.
This requirement for internal inspection would seem to rule out the possibility of using the actual walls of the strut to form the canisters for holding the buoyancy air, since the air would block the signals.
However, it is recognized in the invention, that the main critical area of the strut from the point of view of potential weakening, is the area of the joints between the sections of the strut. It is recognized that an inspection procedure that just examined the joints would be quite adequate, since the possibility of a gradually worsening, detectable fault developing in the main length of the section is very remote.